A known belt type continuously variable transmission controller is configured to perform a belt slip control in which an actual secondary hydraulic pressure is reduced from one during a normal control to slip a belt wrapped around pulleys at a predetermined slip rate by controlling the actual secondary hydraulic pressure on the basis of a multiplier of an oscillation component included in the actual secondary hydraulic pressure and an oscillation component included in an actual gear ratio. This eliminates the necessity for directly detecting the belt slip rate and thereby facilitates the belt slip control. Also, it is known that the estimated accuracy of a belt slip condition is improved by superimposing a predetermined sine wave on a command secondary hydraulic pressure or oscillating the command secondary hydraulic pressure to purposefully oscillate the actual secondary hydraulic pressure and the actual gear ratio (see Patent Document 1, for example).